The direct conversion of radioisotope beta (electron) emissions into usable electrical power via beta emissions directly impinging on a semiconductor junction device was first proposed in the 1950's. Incident beta particles absorbed in a semiconductor create electron-hole-pairs (EHPs) which are accelerated by the built-in field to device terminals, and result in a current supplied to a load. These devices are known as direct conversion semiconductor devices, beta cells, betavoltaic devices, betavoltaic batteries, isotope batteries, betavoltaic power sources, betavoltaic(s) etc. These direct conversion devices promise to deliver consistent long-term battery power for years and even decades. For this reason, many attempts have been made to commercialize such a device. However, in the hopes of achieving reasonable power levels, the radioisotope of choice often emitted unsafe amounts of high energy radiation that would either quickly degrade semiconductor device properties within the betavoltaic battery or the surrounding electronic devices powered by the battery. The radiated energy may also be harmful to operators in the vicinity of the battery.
As a result of these disadvantages, and in an effort to gain approval from nuclear regulatory agencies for these types of batteries, the choice for radioisotopes has been limited to low-energy beta (electron) emitting radioisotopes, such as nickel-63, promethium-147, or hydrogen-3 (tritium). Since promethium-147 is regulated more stringently and requires considerable shielding, whereas nickel-63 has a relatively low beta flux, tritium has emerged as a leading candidate for such a battery device.
Tritium betavoltaic batteries, sometimes referred to as tritium betavoltaic devices or tritium direct conversion devices, have been promoted during the last thirty years. Tritium is listed in various regulatory guideline documents as being in the low toxicity group of radioisotopes producing only low beta energy emissions that can be easily shielded with as little as a thin sheet of paper. Tritium has a long track record of commercial use in illumination devices such as EXIT signs in commercial aircraft, stores, school buildings and theatres. It is also widely used in gun sights and watch dials, making it an ideal power source for the direct conversion devices. Unfortunately, tritium's beta emissions are so low in energy that it is has been difficult to efficiently convert these emissions into usable electrical power for even the lowest power applications, such as powering an SRAM memory to prevent loss of stored data.
Several attempts have been made to produce useful current from a tritium betavoltaic battery. For example, polycrystalline or amorphous semiconductor devices have been considered for tritium betavoltaic batteries based on the assumption that such devices would allow batteries to be fabricated at a reduced cost. It is assumed that these devices could be manufactured in a thin-film like fashion and that tritium could be embedded within the polycrystalline or amorphous devices. However, this approach is extremely inefficient with respect to the beta energy emissions entering the semiconductor (less than about 1%). The main reason for this low semiconductor conversion efficiency is the high dark current that acts as a negative current relative to the current generated by the beta emissions. This high dark current competes with the betavoltaic current produced by collection of EHPs created via the tritium beta particles impinging on the semiconductor. In short, the polycrystalline and amorphous semiconductors have a high number of defects resulting in recombination centers for the EHPs, which in turn significantly reduce the betavoltaic current and lead to very low efficiency for the battery.
The best results for tritium betavoltaics have been achieved with single crystal semiconductor devices. Recent attempts have involved single crystalline semiconductor devices with a tritium source such as a tritiated polymer, aerogel or tritiated metal hydride placed in direct contact with a semiconductor junction device. Single crystalline semiconductors have longer carrier lifetimes and fewer defects resulting in much lower dark currents. Representative efficiencies for tritium betavoltaic batteries were published in a reference text entitled: “Polymers, Phosphors and Voltaics for Radioisotope Microbatteries” edited by K. Bower et al. Single crystal semiconductor devices were exposed to tritium metal hydride sources on top of the semiconductors. Several homojunction (e.g., a conventional junction occurring at the interface between an n-type (donor doped) and p-type (acceptor doped) semiconductor, such as silicon, also referred to as a p-n junction) semiconductor cells were utilized with the following results:
Silicon Cells:                Short Circuit Current=18.1 nA/cm^2        Open Circuit Voltage=0.162        Fill Factor=0.513        Tritiated Titanium Source=0.23 microwatts/cm^2        Efficiency=1.3%        
Aluminum Gallium Arsenide (AlGaAs) Cells:                Short Circuit Current=58 nA/cm^2        Open Circuit Voltage=0.62        Fill Factor=0.751,        Power=27 nW/cm^2        Tritiated Titanium Source=0.48 microwatts/cm^2,        Efficiency=5.6%        
Silicon cells are a preferred choice due to their low cost. However, their low efficiency makes them a poor choice for even the lowest power applications, such as for use in SRAM memory devices. The performance of the AlGaAs homojunction cell is attractive with one of the higher reported efficiencies and would be suitable for powering an SRAM memory device, in particular by stacking of tritiated metal hydride layers and AlGaAs homojunction cells. However, AlGaAs homojunctions cells are difficult to reproduce consistently with uniform dark currents across a semiconductor device due to the oxidation of the aluminum. As a result, AlGaAs is also an expensive option to scale up.
Safety concerns over containment of the tritium based betavoltaic battery have emerged as another obstacle to commercialization of a tritium battery. In commercially available products such as tritium illumination devices (e.g. EXIT signs, gun sights and watch dials), the tritium is in gaseous form and contained within a glass vial. Many accidents involving tritium release due to the breakage of the tritium vials in EXIT signs have caused public concerns and resulted in costly clean-up operations. The use of hermetically sealed packages has reduced these concerns somewhat.
A tritium betavoltaic battery utilizing solid-state tritium metal hydride sources presents a lower exposure risk than gaseous tritium devices. However, the solid-state tritium metal hydride still involves a miniscule amount of tritium release when open to the environment at room temperature. Although several tritium based batteries have been proposed including direct conversion devices built within an integrated circuit, a method of effectively hermetically packaging the battery containing the tritium metal hydride continues to be problematic.
A major obstacle to hermetically sealing this type of battery is the risk associated with using a sealing process that involves high temperatures, i.e., above 200-300° C. At these temperatures tritium is released from the metal hydride, possibly leading to battery failure after sealing, or worse, causing tritium exposure at the sealing manufacturing facility and to the operator of the sealing equipment.
The texturing of a direct conversion semiconductor device to increase the surface area exposed to radiation emission has been proposed several times in the past. For example, on page 282 of the book entitled “Polymers, Phosphors and Voltaics for Radioisotope Microbatteries” edited by K. Bower et al., the use of porous silicon and tritium inserted into porous silicon holes was proposed as a means of increasing the surface area of the semiconductor device by 20 to 50 times, in contrast to the original planar semiconductor surface area.
The following published patent applications and patents each propose a method of increasing the surface area of the semiconductor by textured growth of the semiconductor or a post-growth texturing method:
US Patent Application Publication 2004/0154656
US Patent Application Publication 2007/0080605
U.S. Pat. No. 7,250,323
U.S. Pat. No. 6,949,865
Central to this approach is the hope that an increase in surface area exposed to radioisotope emissions will increase the power per unit volume of the direct conversion semiconductor device. The goal of this approach is to not only reduce the size of the direct conversion device, but also to potentially reduce the cost associated with producing the equivalent surface area in a planar semiconductor device.
The problem with such an approach arises when a relatively low energy radioisotope such as tritium is used. In this case, the incident power is quite small per unit of exposed area and the dark current of the semiconductor device is a very significant factor in the overall efficiency. Unfortunately, alterations to the semiconductor surface risk increasing lattice defects, resulting in a high number of recombination centers for EHPs. This creates a direct conversion semiconductor device with a low open circuit voltage and reduced short circuit current, with a resulting low overall efficiency.
Generally, it is preferable to use a single crystal semiconductor material where device defects are minimized and the dark current is sufficiently low so that power can be efficiently produced.
In accordance with common practice, the various described features are not drawn to scale, but are drawn to emphasize specific features relevant to the invention. Like reference characters denote like elements throughout the figures and text.